Diffusion coefficient of ⁹⁵Nb in α-iron has been determined in the temperature range between 823 and 1163 K by use of a serial sputter-microsectioning technique. The diffusion coefficient of niobium is about two times as large as the self-diffusion coefficient in α-iron. The temperature dependence of the diffusion coefficient, D, in the whole temperature range of α-iron across the Curie temperature (T_C=1043 K) can be expressed by D=(1.40_−1.05^+4.17)×10⁻¹exp\\big[−\\frac(299.7±12.7 kJmol⁻¹)[1+(0.061±0.007)s²]RT\\big] m²s⁻¹ where s is the ratio of the spontaneous magnetization at T K to that at 0 K. The factor 0.061 in the equation is smaller than 0.156 for the self-diffusion, showing that the magnetic effect on the diffusion of niobium in α-iron is smaller than that on the self-diffusion. The activation energy 299.7 kJmol⁻¹ for niobium diffusion in the paramagnetic iron is much higher than 250.6 kJmol⁻¹ for the self-diffusion. Atomic size effect is predominant in the activation energies for the diffusion of transition metal solutes in the paramagnetic α-iron.